Gas chromatography is essentially a physical method of separation in which constituents of a test sample in a carrier gas are adsorbed and desorbed by a stationary phase material in a column. A pulse of the sample is injected into a steady flow of carrier gas. At the end of the column the individual components are separated in time. Detection of the gas provides a time-scaled pattern, which by calibration or comparison with known samples indicates the constituents of the test sample. The main components of such a system are the column, an injector with a mixing chamber for introducing the sample into the carrier gas, a detector at the outlet end of the column, gas controls and a device such as a computer for treating and displaying the output of the detector. An oven may be used to control the temperature of the column, so as to maintain the sample in a volatile state, and to improve the separation of constituents.
Gas chromatographs (GCs) of the current art typically employ electronic pressure control of the column head pressure in order to optimize chromatographic separations and/or run times. On these devices, a low column pressure condition can be sensed when the pressure sensor feedback indicates that a setpoint condition cannot be achieved. When this occurs, an alarm can be indicated to the operator.
Often times this low pressure condition is caused by a gradual and eventual depletion of the carrier gas cylinder contents. This slow and gradual depletion often causes an operator to check the cylinder pressure only on an occasional basis and can result in neglect, which will ultimately cause a low carrier condition to be sensed. Unfortunately, by the time the GC has sensed the low carrier condition it is too late. The analysis underway has been compromised in that an appropriate pressure for accurate retention times or an appropriate inlet pressure during the injection cycle has not been satisfied. The problem is exacerbated when the instrument is running a sequence of samples, since the entire sequence may need to be re-analyzed.
One solution to this problem is to add an additional pressure sensor upstream of the proportional valve in order to sense a low supply line pressure. The regulator delivering a pressure to the GC could be set to higher pressure than that used for the analysis, and the trigger point for an alarm set to correspond to a pressure slightly under the delivered regulator pressure. This would allow sufficient time for an entire sequence of samples to be finished before the operator needed to change cylinders. Unfortunately this solution to the problem adds cost to the electronic pressure control pneumatics and so is generally not implemented. Furthermore, the need to add an additional pressure sensor to the pneumatic control system creates a barrier to implementing this solution in existing gas chromatographs.
Accordingly, there exists a need for a method that overcomes at least some of the above-mentioned limitations.